FCC Chairman Kevin Martin received a nasty letter (PDF) from Comcast VP David Cohen last Friday, full of disappointment over Martin’s choice of words in a Thursday statement responding to the recent Comcast-BitTorrent agreement.

Martin originally said that he was “pleased that Comcast has reversed course and agreed that it is not a reasonable network management practice to arbitrarily block certain applications on its network.”

“Your response,” wrote Cohen, “was perplexing; it repeated erroneous characterizations of Comcast’s network management practices and disclosure policies that we have taken great pains to clarify on multiple occasions.”

“As we have unambiguously stated on the record, Comcast’s customers have been, are, and will continue to be free to access any lawful Internet content and to use any application and service of their choice, including those that utilize peer-to-peer (‘P2P’) protocols.”

Cohen called his company’s policy a “delaying” act, allowing Comcast to better allocate what it claims to be scarce resources: a system that is “reasonable,” “minimally intrusive” and switched on “only when necessary to prevent network congestion.”

“These practices do not deny our customers’ access to these applications and services, but rather facilitate and enable the use of these and countless other applications and services by all of our customers,” he said.

The trouble with Martin’s “characterization,” as well as Cohen’s response, is that both are accurate in some ways and inaccurate in others: when Comcast’s “data discrimination” was originally discovered in October 2007, it was found to adversely affect a large number of protocols, including BitTorrent, e-Donkey, Gnutella, and even Lotus Notes.

For BitTorrent downloads, Comcast’s “delaying” – which manifests as Comcast’s servers injecting spoofed “reset” messages inside a given BitTorrent connection, causing both ends to disconnect – would only kick in after the user’s torrent switched to “seeding” mode, which occurs after it finishes downloading. This means that while the file in question wasn’t truly “blocked” per se, Comcast is responsible for hurting the health of the overall swarm (the collection of all computers involved in a given BitTorrent download) and, in some cases, rendering BitTorrent downloads to be unacceptably slow for users both in and out of Comcast’s network.

Martin also questioned Comcast’s timeline, wondering why its current policy is set to “continue throughout the country until the end of the year and in some markets, even longer.”

“It is not at all obvious why Comcast couldn’t stop its current practice of arbitrarily blocking its broadband customers from using certain applications,” he stated.

 “We just cannot turn off our current system overnight,” replied Cohen, “ and put our customers at risk of network congestion. For the benefit of our customers, it is essential that the migration be appropriately timed, a reality that BitTorrent and numerous commentators acknowledge.”

While the Comcast-BitTorrent agreement has indeed received wide praise, it is not without its skeptics: Vuze, Inc., makers of the popular BitTorrent client Azureus Vuze and the Vuze legal media portal, also filed a brief with the Commission, arguing for government regulation regardless of what becomes of Comcast’s agreement (PDF).